The present invention relates to an interferometric method and apparatus for measuring physical parameters.
Distributed temperature sensors (DTS) are known in which an optical fiber is deployed through a region of interest, light is launched into the fiber, and backscattered optical radiation is used to determine the temperature variation along the fiber. Such distributed temperature sensors have proven to be useful in oil production at determining, for example, flow contributions from different oil producing zones, identifying the location of the ingress of other fluids and the position at which gas breaks out. However, the interpretation of temperature data relies on the contrast between the thermal profile of a flowing well and the geothermal gradient. In those cases where the geothermal gradient is small (e.g. in a horizontal well), then the information may be obtained by inducing a thermal event, which can be tracked, thus revealing the local flow rate. This approach works well in slow-flowing wells. For much higher flow rates some information can be gained from frictional heating effects, but these are very small and the temperature resolution of DTS systems, especially in extended reach-drilling wells, is marginal. The resolution limit of the DTS is of order 0.1° C. for long measurement times. This limits the flow resolution to typically 5000 bpd in a 7″ bore.
However, it is thought that much more information would be available if the temperature could be measured to a higher resolution, e.g. 0.01° C. or better.
Moreover, the DTS is relatively slow when set up for best resolution at long range. It is thought that a fast, high resolution temperature measurement might allow either natural, or induced, thermal fluctuations to be tracked and thus provide a more direct indication of flow in fast-flowing wells.
Other measurands would also add to the picture of the well. For example, a measurement of the pressure distribution along the producing interval would allow a precise determination of flow if allied to a density measurement (which might also be obtained from pressure measurements) and some form of mixing. However, pressure sensors with adequate performance for flow determination (e.g. permanent quartz gauges) are expensive and have only limited multiplexing capability.
A solution to these difficulties is now sought on the basis of interferometric interrogation of reflector arrays. This approach is commonly used in fiber-optic hydrophones, which have been deployed for naval applications, as known from Dakin, J. P., Wade, C. A. and Henning, M., 1984, “Novel optical fibre hydrophone array using a single laser source and detector”, Electron. Letts, vol. 20, pp 53-54, and from Dakin, J. P. and Wade, C. A., 1984, “Optical Fibre Hydrophone Array—recent progress”, Proc. 2nd Optical Fibre Sensors Conference, Stuttgart, pp 375-379.
In acoustic measurements, the objective is only to determine an AC signal which is the change in relative phase of the optical reflections from adjacent reflectors in the array. Owing to the AC nature of the signal processing, many problems that are encountered in static measurements are circumvented.
In an interferometric array, reflectors are spaced by typically a few tens of meters. In hydrophones, the AC signal is obtained by exposing the fiber to strain when an acoustic wave is present; the means of transducing a pressure wave into strain might involve wrapping a fiber on an air-backed mandrel. The fiber itself has a moderate sensitivity to pressure waves, even in the absence of such a mandrel. This signal is detected in the surface instrumentation in the form of a variation of the phase as a function of time. This phase signal, which might stretch over more than 2π, conveys information on the instantaneous pressure. The relative phase is, of course, directly related to the optical path length in the section of fiber between the reflectors. However, in acoustic arrays, only changes in the phase are considered. The absolute optical path length is not important and is not measured.
However, if the absolute optical path length between reflectors is known to interferometric precision (i.e. a small fraction of the wavelength of light), then the physical parameters on which it depends (e.g. temperature and strain) can be deduced very precisely. In particular the optical path length depends on the physical length of optical fiber between reflectors and the refractive index separating the reflectors, both of which are sensitive to a variety of external influences. For example, the length changes through thermal expansion and the refractive index varies with temperature through the thermo-optic effect. Likewise, strain obviously affects the length of the fiber, but also the refractive index through the stress-optical effect. In an optical fiber, the index in question, the so-called effective index, takes a value intermediate between that of the core and the cladding, depending on certain waveguiding effects; the latter effects may themselves be dependent on an external physical parameter, but this is thought to be a second order effect.
One problem is that interferometric sensors provide a periodic response to the measurand and their answer is thus ambiguous. Thus the output of the sensor, as a function of the measurand, is nominally sinusoidal, each period being referred to as a fringe. In the typical path length between reflectors, there exist a very large number of fringes and it is then difficult to determine on which fringe the interferometer is presently sited.
As an example, take a wavelength of 1550 nm and a spacing of 10 m between reflectors. The number of fringes between the reflections is of order 1.863×107. If we assume that the sensor can resolve 1/1000 of a fringe, then the resolution in determining the optical path length would be some 1:1.863×1010. How this resolution translates into a determination of the quantity of interest (e.g. temperature) will of course depend on the sensitivity of the optical path length to that parameter. In the above example, the temperature resolution would be better than 1 mK. However, unless the measurement system is able to determine the fringe order, this very high resolution is meaningless. In contrast, an acoustic sensor merely needs to track changes in the fringe order over an acoustic period (a few ms).
The key to the present approach involved in determining the absolute optical length between reflectors is therefore to resolve the fringe order ambiguity over such a large number of fringes.